1. Field of the Invention
The present invention relates to an antenna for a mobile phone, and more particularly to an antenna for a mobile phone having an improved structure, which reduces specific absorption rate (hereinafter referred to SAR) with respect to an electronic circuit board, which is connected to the antenna and into which electromagnetic waves are absorbed as power is fed through the antenna.
2. Description of the Related Art
Recently, as wireless mobile terminals are widely used and various electronic industries have developed rapidly, devices for shielding electromagnetic waves that are harmful to the human body or reducing SAR have been developed. SAR is electromagnetic energy (W/Kg) absorbed into the human body per unit mass when a person uses a mobile terminal. SAR is a yardstick for representing an amount of electromagnetic waves absorbed into the human body and is used as a parameter for a human protection standard.
Generally, a mobile terminal is used closely to the human body, so SAR is affected by a near field region of waves radiated from an antenna of the mobile terminal. Therefore, SAR is closely related to the transmitting power of the mobile terminal, an antenna characteristic, and a structure of the mobile terminal.
Various methods have been developed to reduce SAR. For example, a directional antenna and a separate conductive plate are used to shield electromagnetic waves, and a wave absorption member is inserted into the mobile terminal.
SAR is measured as shown below in Equation 1SAR=σ/2ρ|E i|2  Equation 1
wherein, σ is an electric conductivity (S/m) of human tissue making contact with electromagnetic wave, Ei is an intensity (V/m) of am electric field penetrated into the human body, and ρ is a density of human tissue making contact with the electromagnetic wave.
If a high SAR is measured, it has a bad influence on the human body. Therefore, many countries have set a SAR reference value to a head of the human body in such a manner that the SAR may not exceed the reference value. In Korea, the SAR reference value is 1.6 (W/Kg), which is identical to that of the U.S.A. However, in Japan and the E.U., the SAR reference value is 2.0 (W/Kg).
Conventionally, an antenna of a mobile terminal is aligned as far from the human body as possible when a user makes communication using the mobile terminal in order to reduce SAR by the human body. FIGS. 1 and 2 illustrate conventional mobile terminals having antennas positioned away from the human body when making communication through the mobile terminals.
Referring to FIG. 1, a connection member 5 is installed at an outer portion of the mobile terminal. An antenna 3 is connected to the connection member 5 in such a manner that the antenna 3 is installed at the outer portion of the mobile terminal while being remotely positioned from a printed circuit board 10 by a predetermined distance. In this case, due to the connection member 5, the SAR according to current introduced into the printed circuit board 10 is about 1.89 (W/Kg). The SAR has been measured through an experiment and has a tolerance about ±0.1 (W/Kg).
Referring to FIG. 2, a connection member 9 is installed in the mobile terminal. In addition, an antenna 7 is installed at a rear portion of the mobile terminal through the connection member 9 while being spaced by a predetermined distance from a printed circuit board 10. In this case, due to the connection member 9, the SAR according to current introduced into the printed circuit board 10 of the mobile terminal is about 0.82 (W/Kg). The SAR is measured through an experiment and has a tolerance about ±0.1 (W/Kg).
Although the above structures can ensure a predetermined distance between the antenna and the mobile terminal, current also flows through the printed circuit board 10 of the mobile terminal for feeding power to the antenna. Accordingly, the printed circuit board 10 acts as an antenna. Thus, the predetermined distance ensured between the antenna and the mobile terminal is meaningless. That is, even though a predetermined distance is set between the antenna and the mobile terminal, power fed to the antenna is flowing back into the printed circuit board, thereby generating electromagnetic waves in the printed circuit board.
Therefore, although the above structures ensure a sufficient distance between the antenna and the mobile terminal for reducing SAR, a real value of SAR exceeds the above experimental values due to current flowing through the printed circuit board.
In order to prevent current from flowing through the printed circuit board from the antenna, a method has been suggested for installing a balun 14 in the form of a choke between an antenna 12 and a printed circuit board 10 as illustrated in FIG. 3. However, according to the above method, the antenna 12 is spaced far from the mobile terminal due to the balun 14, thereby deforming an outer appearance of the mobile phone. Therefore, it is impractical to mount the antenna 12 by using the balun 14.